Relatively high value capacitors (e.g., 100 pF or more) generally require a relatively large plate area and are more expensive to fabricate than lower value capacitors. Due to the large physical size of high value capacitors, such high value capacitors cannot be easily incorporated into an integrated circuit and thus must be connected external to an integrated circuit.
Capacitance is calculated using the equation: EQU C=AK.epsilon..sub.O /t,
where
C is the capacitance PA1 .epsilon..sub.O is the permittivity of empty space (8.85.times.10.sup.-2 coul.sup.2 /newton.sup.2 -m.sup.2) PA1 K is the dielectric constant (3.9 for SiO.sub.2) PA1 A is the plate area, and PA1 t is the dielectric thickness.
As seen from the above equation, capacitance can only be significantly increased by either increasing the plate area or decreasing the thickness of the dielectric between the plates. The dielectric thickness is limited by the process used, the operating voltage, and the difficulty in achieving a consistent thin dielectric thickness over a large area.
High value capacitors may be required in circuits for achieving a desired operating bandwidth of an AC coupled amplifier, for filtering, and for other uses well known to those skilled in the art.
Thus, what is needed is a circuit and method for achieving a high capacitance value in a circuit which does not require a capacitor having a large area.